The present invention relates to the fabrication of high density compacts of various titanium base alloys with high strength at high temperatures. Such compacts may include articles of various shapes and sizes prepared by plasma spray deposition. Following deposition, the compacts are consolidated to high density to achieve the properties of wrought articles.
It is known that silicon carbide filaments can be formed with great strength and with high temperature tolerance. It is also known that titanium metal foils have been used in connection with SiC filaments to produce SiC reinforced composites in which the SiC filaments are embedded in a sheet of titanium alloy made up of a number of layers of foil. Such SiC reinforced titanium alloy composites have been identified as potential high strength materials, that is materials which have high strength to weight ratio. Such materials are deemed to be attractive for use in future aircraft engines having high thrust to weight ratios and in wing structures of transatmospheric vehicles. It is anticipated that such titanium alloy matrix composites and laminates will find application in wound rotors and in casings and is other intermediate temperature high stress applications.
Under present practice, titanium alloy composites have been fabricated by rolling the desired titanium alloy ingot to about 0.008 to 0.010 inch thick sheet. The sheet is employed as alternate layers in a lay up of titanium alloy sheet and an array of parallel SiC fibers held together with very fine Ti ribbon to form a preconsolidated assembly. The assembly is then consolidated by hot pressing or hot isostatic pressing (HIPing).
Fabrication of such thin titanium alloy sheets for formation of such a composite can be very costly. This is particularly so if the titanium alloy is not ductile at room temperature. One alloy which lacks such room temperature ductility is niobium modified Ti.sub.3 Al. This alloy can only be rolled to foils of about 0.020 inch thick. To obtain thinner sheet requires that the thicker sheet be electrochemically machined to the desired thickness. If the final desired thickness is 0.010 inch, then about half of the original material is lost.
Novel and unique structures are formed pursuant to the present invention by plasma spray deposit of titanium base alloys including titanium-aluminum intermetallic compounds employing RF plasma spray apparatus either with or without reinforcing filaments.
The formation of plasma spray deposits of titanium and of alloys and intermetallic compounds of titanium present a set of processing problems which are unlike those of most other high temperature high strength materials such as the conventional superalloys. A superalloy such as a nickel base or iron base superalloy can be subdivided to relatively small size particles of -400 mesh (about 37 .mu.m) or smaller without causing the powder to accumulate a significant surface deposit of oxygen. A nickel base superalloy in powder form having particle size of less than -400 mesh will typically have from about 200 to about 400 parts per million of oxygen. A powdered titanium alloy by contrast will typically have a ten fold higher concentration of oxygen. A powdered titanium alloy of -400 mesh will have between about 2000 and 4000 ppm of oxygen.
Moreover, titanium alloy powder of less than -400 mesh size is recognized as being potentially pyrophoric and as requiring special handling to avoid pyrophoric behavior.
It is also recognized that the room temperature ductility of titanium alloys decreases as the concentration of oxygen and of nitrogen which they contain increases. It is accordingly important to keep the oxygen and nitrogen content of titanium base alloys at a minimum. This can be very difficult for very finely divided powders of titanium base alloys.
Prior art plasma spray technology is based primarily on use of direct current plasma apparatus. It has been recognized that most as-sprayed plasma spray deposits of the superalloys such as nickel and iron base superalloys have had relatively low ductility and that such deposits when in their as-deposited sheet form can be cracked when bent through a sufficiently acute angle due to the low ductility.
I have discovered that RF plasma apparatus is capable of spraying powder of much larger particle size than the conventional DC plasma apparatus. I have discovered that particle sizes at least three times larger in diameter than those conventionally employed in DC plasma spray apparatus may be successfully employed. The particle size may be as high as 100 .mu.m to 250 .mu.m and larger and as large as 10X as large as the -400 mesh powder previously employed in DC plasma spray practice.
This possibility of employing the larger powder particles is quite important for metal powders such as titanium which are subject to reaction and absorption of gases such as nitrogen and oxygen on their surfaces. One reason is that the surface area of particles relative to their mass decreases inversely as their diameters. Accordingly, a three fold increase in particle diameter translates into a three fold decrease in particle surface area to volume. I have discovered that one result is that RF plasma spray deposited structures of titanium base alloys made with the aid of larger particles have lower oxygen content than might be expected based on knowledge of prior art practices.
As used herein, the term titanium base alloy means an alloy composition in which titanium is at least half of the composition in parts by weight when the various alloy constituents are specified, in parts by weight, as for example in percentage by weight.
A titanium-aluminum intermetallic compound is a titanium-base alloy in which titanium and aluminum are present in a simple numerical atomic ratio and the titanium and aluminum are distributed in the composition in a crystal form which corresponds approximately to the simple numerical ratio such as 3:1 for Ti.sub.3 Al; 1:1 for TiAl and 1:3 for TiAl.sub.3.
Ti.sub.3 Al compositions have use temperatures of up to about 1400.degree. F as compared to the use temperatures of titanium alloys such as Ti-6Al-4V of up to about 1000.degree. F. The use temperatures of TiAl is in the 1700.degree.-1800.degree. F range. Ti-6Al-4V a is titanium base alloy containing 6 weight percent aluminum, 4 weight percent vanadium and the balance is titanium.
As used herein, the term "sub-compact" means a plasma spray as-deposited structure having overall thickness dimensions greater than that of a foil. It may contain or include filament or other reinforcement as a composite or it may include only plasma spray deposited metal. The asdeposited metal of a sub-compact has internal voids and a density below 100%. The compact prepared pursuant to the present invention has a high density closed porosity outer surface layer extending essentially over the complete outer surface of the sub-compact.